Rice oil contaminated with PCB-based heat exchange fluid in Japan (Yusho oil) has been analyzed primarily for content of polyhalogenated aromatics. This cursory examination has confirmed the presence of transformation/rearrangement products of PCB's among other products suggesting this would be a complex toxicological mixture which is not representative of PCB's. Similarly, preliminary examination of new and used transformer fluids from this country indicate that such PCB-based fluids do undergo considerable alterations and rearrangements during use. The complex mixtures generated would be expected to have complex toxicological behavior as well.